The present invention relates to a method and an apparatus for synchronously controlling the printing speed of a belt-type printing machine.
Conventionally, a single faced corrugated board is manufactured by forming corrugations in a medium web of paper and then by gluing a liner web of paper onto the corrugation crests on one of the sides of the medium web. A liner web of paper is glued onto the corrugated crests on the other side of the medium web to form a double faced corrugated board. In these cases it is preferable to print the desired pattern on the liner or liners prior to formation of the corrugated board in order to obtain an excellent printing appearance and to avoid lowering of the compressive strength of the corrugated board, which would occur due to passage of the corrugated board through printing machine.
In a conventional rotary printing machine, the length of the printed area of the web is limited by the circumferential length of the plate drum and therefore it is impossible to print a pattern of long length exceeding the circumferential length of the plate drum. In addition, a large blank or wasted space remains between adjacent printed areas once every rotation of the plate drum when the printing is carried out by a printing plate having a shorter length than the circumferential length of the plate drum.
For solving these problems, a belt-type printing machine has been recently proposed. As shown in FIG. 6, the belt-type printing machine of prior art usually comprises a plate cylinder 10, a tension roll 12 arranged for adjusting a distance between the plate cylinder 10 and the tension roll 12, and an endless printing belt 14 of synthetic resin film detachably wound around the plate cylinder 10 and the tension roll 12 and travelling therearound in one direction. A printing plate or plates 16 have characters or patterns formed thereon and are affixed on the printing belt 14. Accordingly, it is able to print the characters or patterns on a continuous sheet of paper 20 passing it between the plate cylinder 10 and an impression cylinder 18 arranged close to the plate cylinder 10. According to the belt-type printing machine, the length of printed patterns is not limited by the circumferential length of the plate cylinder 10 and therefore a printed area of any desired length can be conveniently made on the continuous sheet of paper merely by changing the length of either the endless printing belt 14 or the printing plate 16.
As clearly shown in FIG. 6, the belt-type printing machine of prior art is provided with sprockets which are mounted on the opposite ends of the plate cylinder 10 and have a plurality of sprocket pins 10a adapted to engage perforations 14a formed in the printing belt 14 along its longitudinal edges. During the rotation of the plate cylinder 10, the sprocket pins 10a act not only to prevent the endless printing belt 14 from its lateral movement on the plate cylinder 10 but also to drive the belt 14.
However, a slight speed differential tends to arise between the travelling speed of the printing belt 14 and the speed of rotation of the plate cylinder 10 (more in detail the peripheral velocity of the plate cylinder 10), this differential being generally increased cumulatively. This causes misalignment and interference between the sprocket pins 10a and the perforations 14a of the printing belt 14, which damages the perforations 14a and greatly lowers the durability of the belt 14. In addition, the speed differential between the belt 14 and the plate cylinder 10 (the speed of the printing cylinder 10 is same as a line speed of a series of printing machines) sometimes causes an unevenness of color and/or a shear when carrying out a multiple-color printing by using a plurality of type printing machines arranged in series.
Such a phenomenon of causing the speed differential is remarkably found especially when a large printing plate 16 is affixed onto the printing belt 14. The reason why the speed differential is caused will be inferred from following considerations. According to the belt-type printing machine shown in FIG. 6, the endless printing belt 14 is driven not only by a frictional driving force between the belt 14 and the plate cylinder 10 but also by a driving force due to an engagement of the sprocket pins 10a with the perforations 14a of the belt 14. With this arrangement, there would arise a differential between driving forces given by the frictional force of the plate cylinder 10 and the engagement of the sprocket pins 10a with the perforations 14a since the flexibility of the printing belt 14 is lowered owing to the affix of the large printing plate 16 onto the printing belt 14. Such a driving force differential will cause the speed differential as set out above. Other various factors such as an unevenness in thickness of the endless printing belt 14, the printing plate 16 and an adhesive tape (not shown) for affixing the printing plate 16 onto the printing belt 14 will complicatedly act each other and give influences to the speed differential between the printing belt 14 and the plate cylinder 10. It has been found that such a speed differential develops a tendency of both increasing and decreasing the travelling speed of the endless printing belt 14.